It has long been an object of chemists to extract and commercially utilize the lignin recovered from natural ligno-cellulose materials such as wood. This objective has been highlighted in recent years with the public cognizance of an energy crisis. Climbing prices for oil and natural gas have drawn attention and effort to method of exploiting the lignin ingredient of wood as a source of plastics feed stock.
In the field of polyurethane chemistry, many attempts have been made to manufacture a high quality polyurethane product from various lignin sources and derivatives.
Lignin, which is a by product of the pulp and paper industry, is available in large quantities. Because of its complex nature and undefined chemical structure, however, it has not been considered as a valuable chemical intermediate. In fact, because of its complicated structure, it has created many disposal problems.
Presently, lignin is used almost exclusively as fuel to power the evaporators of the chemical recovery processes and liquir concentration system of pulpmills. Applicants share the belief of other lignin chemists that lignin can achieve a higher value as industrial raw material than as a fuel. At present prices of $11 per barrel of Bunker-6 fuel oil, the fuel value of kraft lignin in black liquor amounts to only 1.98 cents per pound which is about 50% less than crude oil. In general, these are four distinctly different lignin utilization schemes.
First, lignin may assume a role as "feed stock" for low molecular weight materials such as phenols which are base chemicals of many products. However, a competitive advantage of lignin over some petroleum or other fossil materials would be best insured by converting it into polymeric materials which retain lignin's structural characteristics. Secondly, polymer modification, rather than breakdown and resynthesis, appears to be another promising approach to the utilization of lignin. Fertilizers, ion exchange resins, and polyurethane products, to name a few, are candidates for such lignin outlets. A third possibility presents itself through a rapidly developing microbiological engineering technology, which views lignin as a natural "protein-precursor". Finally, lignin is particularly valuable if retained in high yield pulp.
Lignin is the second most abudant substance in wood, exceeded only by cellulose. It occurs in amounts ranging from 20 to 35% of natural wood content depending on the species, as well as in other parts of the tree such as leaves, shoots, stalks, branches, trunks, and roots. Lignin is thought of as a light brown amorphous "cement" that fills the gaps between the long, thin polysaccharide fibers in the cell walls and binds them together. The role of lignin in gluing the plant fibers together can be compared to that of the polyester resin which is used to strengthen the fiberglass webbing of an automobile body.
Paper producers use various alkaline and/or acid chemicals to dissolve lignin and to liberate the fibers for papermaking. For them, the lignin is an undersirable wood component.
Presently, there are two main methods in use for removing lignin from wood. The first method is known as the sulfite process, wherein the wood is cooked with various salts of sulfurous acid. In the second method which is known as the kraft process, wood is cooked with a solution containing sodium hydroxide and sodium sulfide. The dark solutions of the degraded lignin which are dissolved out from the wood are commonly known as "spent sulfite liquor" in the sulfite process, and "black liquor" in the kraft process, respectively. These spent pulping liquors are usually concentrated for use as fuel, and for the recovery of certain pulping chemicals.
The unique chemical and physical properties of the lignin-derived polymer has given it a place among specialty polymer applications such as dispersants, emulsifiers and phenol-based adhesives. For these purposes a part of the lignin is recovered from the spent pulping liquors. The reduction in heat value of these liquors is thereby made up with other fuels.
The limited commercial utilization of lignin is occasioned principally by reason of its physical and chemical characteristics. Thus, lignin is not resistant to water and is soluble in alkaline solutions. Moreover, it is a nonthermosetting thermoplastic which tends to disintegrate if heated above 200.degree. C and which, if formable at all from the amorphous powdered condition is recovered, merely provides a crumbly mass of little or no strength.
As can easily be extracted from the foregoing one of the goals in lignin chemistry is to develop alternate uses for lignin whereby this unique renewable natural polymer can be disposed of more profitably than it is at the present time.
Lignin is composed of carbon, hydrogen and oxygen in different proportions. Its basic building units are phenylpropanes which are interconnected in a variety of ways by carbon-carbon and carbon-oxygen bonds, giving lignin a complicated three-dimensional structure. The molecular weight of lignin varies with its method of isolation, and its source. Lignin from a sulfite pulping process generally has an average molecular weight of about 20-100 thousand. Lignin from kraft pulping processes on the other hand has a lower average molecular weight which ranges from 1.5-5 thousand.
Another characteristic of lignin is that the number of hydroxyl groups per given weight increases as the molecular weights of the lignins decrease. Because low molecular weight lignin possesses a higher percentage of hydroxyl groups, it has a higher potential to react with oxyalkylating modification reagents such as ethylene glycol, ethylene oxide, propylene oxide and others. Apart from the reactive hydroxyl sites, lignin possesses various carbonyl, carboxyl, aldehyde and ethylene groups which provide additional sites for other modification reactions.
The chemical pulping agents generally referred to above degrade lignin into a condensed spherical core polymer with reduced activity when compared with that which exists in its naturally occurring state. This is possibly due to the higher surface tension spherical form which may cause the lignin to become a hard-to-modify material. Notwithstanding this negative factor, lignin has been used in various products because of its availability.
As alluded to above, in general plastics applications, there are two possible ways to utilize lignin. First of all lignin may be degraded into a low-molecular weight compounds commonly referred to as feed stocks and then reconverted to various synthetic polymers. Secondly, lignin may be used in its natural high-molecular weight state following suitable chemical modification. Such modifications may utilize and act upon any one of the many functional groups present in the complex lignin polymer.
While these general approaches appear simple, they are complicated as well as unpredictable in both application and intended result.
In the more restrictive field of polyurethane chemistry, for example, the objective has long been to develop a high quality polyurethane product from lignin sources which is light in color, non-odorous, and of low density while possessing high compressive strength and a low water absorption characteristic. No prior art attempts to satisfy this multi-purpose commercial objective have been successful.